A. Definitions
As used in this specification and in the claims, the following terms shall have the following meanings:
(1) Telecentric
Telecentric lenses are lenses which have at least one pupil at infinity. In terms of principal rays, having a pupil at infinity means that the principal rays are parallel to the optical axis (a) in object space, if the entrance pupil is at infinity, or (b) in image space, if the exit pupil is at infinity.
In practical applications, a telecentric pupil need not actually be at infinity since a lens having an entrance or exit pupil at a sufficiently large distance from the lens' optical surfaces will in essence operate as a telecentric system. The principal rays for such a lens will be substantially parallel to the optical axis and thus the lens will in general be functionally equivalent to a lens for which the theoretical (Gaussian) location of the pupil is at infinity.
Accordingly, as used herein, the terms “telecentric” and “telecentric lens” are intended to include lenses which have a pupil at a long distance from the lens' elements, and the term “telecentric pupil” is used to describe such a pupil at a long distance from the lens' elements. For the projection lenses of the invention, the telecentric pupil distance will in general be at least about 20 times the lens' focal length.
(2) Effective Back Focal Length
The effective back focal length (BFL) of a projection lens/pixelized panel combination is the distance between the front surface of the pixelized panel and the vertex of the back surface of the rearward-most lens element of the projection lens which has optical power when (1) the image of the pixelized panel is located at infinity and (2) the projection lens is located in air, i.e., the space between the rearward-most lens element of the projection lens and the pixelized panel is filled with air as opposed to the glasses making up the prisms, beam splitters, etc. normally used between a projection lens and a pixelized panel.
(3) Q-Value
As described in J. Hoogland, “The Design of Apochromatic Lenses,” in Recent Development in Optical Design, R. A. Ruhloff editor, Perkin-Elmer Corporation, Norwalk, Conn., 1968, pages 6-1 to 6-7, the contents of which are incorporated herein by reference, Q-values can be calculated for optical materials and serve as a convenient measure of the partial dispersion properties of the material.
Hoogland's Q-values are based on a material's indices of refraction at the e-line (546 nanometers), the F′ line (480 nanometers), and the C′ line (643.8 nanometers). The wavelengths used herein, both in the specification and in the claims, are the d line (587.56 nanometers), the F line (486.13 nanometers), and the C line (656.27 nanometers).
More particularly, as described in Hoogland, the Q-value for a lens element is determined using the indices of refraction Nd, NF, and NC of the material making up the element at the d, F, and C lines, respectively, and the equation:Q=(y−yn)×106 where y is given by:y=(NF−Nd)/(Nd−1)and yn is determined from an equation of the form:yn=ax+b evaluated at the x-value for the material making up the lens element, where x is given by:x=(NF−NC)/(Nd−1)and a and b are determined using x and y values for SK16 and SF2.
(4) V-Value
V-values (also known as Abbe constants) are for the d, F, and C lines and are given by:V=(Nd−1)/(NF−NC)
(5) N-Value
Indices of refraction (N-values) are for the d-line (587.56 nanometers) in Table 6. All focal lengths and other calculated values which depend on a single value for the index of refraction for individual elements are for the e-line (546.1 nanometers).
(6) Lens Element F-Number
The f-number of a lens element (F/#element) is the ratio of the element's focal length to its maximum clear aperture, i.e., the largest of its long conjugate and short conjugate clear apertures.
(7) Vignetting
The vignetting of a projection lens in percent is defined as 100 minus 100 times the ratio, in the long conjugate focal plane, of the illuminance at the full field to the illuminance on-axis at the projection lens' working f-number. Since projection lenses normally do not include an adjustable iris and are used “wide open,” the working f-number will typically be the full aperture f-number.
B. Projection Systems
Image projection systems are used to form an image of an object, such as a display panel, on a viewing screen. Such systems can be of the front projection or rear projection type, depending on whether the viewer and the object are on the same side of the screen (front projection) or on opposite sides of the screen (rear projection).
FIG. 6 shows in simplified form the basic components of an image projection system 17 for use with a pixelized imaging device (also known in the art as a “digital light valve”). In this figure, 10 is an illumination system, which comprises a light source 11 and illumination optics 12 which transfer some of the light from the light source towards the screen, 13 is the imaging device, and 14 is a projection lens which forms an enlarged image of the imaging device on viewing screen 15. For front projection systems, the viewer will be on the left side of screen 15 in FIG. 6, while for rear projection systems, the viewer will be on the right side of the screen.
For ease of presentation, FIG. 6 shows the components of the system in a linear 5, arrangement. For a reflective imaging device and, in particular, for a DMD imaging device of the type with which the present invention will typically be used, the illumination system is arranged so that light from that system reflects off of the imaging device, i.e., the light impinges on the front of the imaging device as opposed to the back of the device as shown in FIG. 6. Also, for such imaging devices, one or more prism assemblies (see “PR” in FIGS. 1A, 2A, 3A, 4A, 4B, and 5) will be located in front of the imaging device and will receive illumination light from the illumination system and will provide imaging light to the projection lens. In addition, for rear projection systems which are to be housed in a single cabinet, one or more mirrors are often used between the projection lens and the screen to fold the optical path and thus reduce the system's overall size.
The linear arrangement shown in FIG. 6 can also be modified in the case of a transmissive imaging device. Specifically, in this case, the optical path between the imaging device and the screen can include two folds to reduce the overall size of the cabinet used to house the system, e.g., a first fold mirror can be placed between imaging device 13 and projection lens 14 and a second fold mirror can be placed between the projection lens and screen 15.
Image projection systems preferably employ a single projection lens which forms an image of: (1) a single imaging device which produces, either sequentially or simultaneously, the red, green, and blue components of the final image; or (2) three imaging devices, one for red light, a second for green light, and a third for blue light. Rather than using one or three imaging devices, some image projection systems have used two or up to six imagers. Also, for certain applications, e.g., large image rear projection systems, multiple projection lenses are used, with each lens and its associated imaging device(s) producing a portion of the overall image. Irrespective of the details of the application, the projection lens generally needs to have a relatively long effective back focal length to accommodate the prisms, beam splitters, and other components normally used with pixelized panels. In the preferred embodiments of the present invention, a single projection lens is used to form an image of a single imaging device, e.g., a DMD panel. For this application, the projection lens needs to have a relatively long effective back focal length to accommodate the one or more prism assemblies used with such a panel (see above).
A particularly important application of projection systems employing pixelized panels is in the area of rear projection systems which can be used as large screen projection televisions (PTVs) and/or computer monitors. To compete effectively with the established cathode ray tube (CRT) technology, projection systems based on pixelized panels need to be smaller in size and lower in weight than CRT systems having the same screen size.
C. Optical Performance
To display images having a high information content (e.g., to display data), a microdisplay must have a large number of pixels. Since the devices themselves are small, the individual pixels are small, a typical pixel size ranging from 17μ for DMD displays to approximately 8μ or even less for reflective LCDs. This means that the projection lenses used in these systems must have a very high level of correction of aberrations. Of particular importance is the correction of chromatic aberrations and distortion.
A high level of chromatic aberration correction is important because color aberrations can be easily seen in the image of a pixelized panel as a smudging of a pixel or, in extreme cases, the complete dropping of a pixel from the image. Lateral color, i.e., the variation of magnification with color, is particularly troublesome since it manifests itself as a decrease in contrast, especially at the edges of the field. In extreme cases, a rainbow effect in the region of the full field can be seen.
In projection systems employing CRTs a small amount of (residual) lateral color can be compensated for electronically by, for example, reducing the size of the image produced on the face of the red CRT relative to that produced on the blue CRT. With a pixelized panel, however, such an accommodation cannot be performed because the image is digitized and thus a smooth adjustment in size across the full field of view is not possible. A higher level of lateral color correction, including correction of secondary lateral color, is thus needed from the projection lens.
The use of a pixelized panel to display data leads to stringent requirements regarding the correction of distortion. This is so because good image quality is required even at the extreme points of the field of view of the lens when viewing data. As will be evident, an undistorted image of a displayed number or letter is just as important at the edge of the field as it is at the center.
Moreover, projection lenses are often used with offset panels. In particular, in the case of DMDs, an offset is typically needed to provide the appropriate illumination geometry and to allow the dark-field light to miss the entrance pupil of the lens. This dark-field light corresponds to the off position of the pixels of the DMD.
When a panel is offset, the distortion at the viewing screen does not vary symmetrically about a horizontal line through the center of the screen but can increase monotonically from, for example, the bottom to the top of the screen. This effect makes even a small amount of distortion readily visible to the viewer.
Low distortion and a high level of color correction are particularly important when an enlarged image of a WINDOWS type computer interface is projected onto a viewing screen. Such interfaces with their parallel lines, bordered command and dialog boxes, and complex coloration, are in essence test patterns for distortion and color. Users readily perceive and object to even minor levels of distortion or color aberration in the images of such interfaces.
In addition to high levels of color and distortion correction, projection lenses for use with pixelized panels need to have low levels of ghost generation, especially when the pixelized panel is of the reflective type, e.g., a DMD or reflective LCD.
As known in the art, ghosts can be generated by image light reflecting back towards the object from one of the lens surfaces of a projection lens. Depending upon the shape of the lens surface and its location relative to the object, such reflected light can be re-reflected off of the object so that it reenters the projection lens and is projected onto the screen along with the desired image. Such ghost light always reduces contrast at least to some extent. In extreme cases, a second image can actually be seen on the screen.
Because the operation of DMDs and reflective LCDs depend upon their ability to reflect light efficiently, projection systems employing panels of these types are particularly susceptible to ghost problems. Ghosts can also be generated by light reflecting backwards off of one lens surface and then being re-reflected in a forward direction by a second lens surface. When reflective pixelized panels are used, ghosts generated by reflections from two lens surfaces are generally less troublesome than ghosts generated by a lens surface/pixelized panel combination.
As is well-known in the art, field dependent aberrations increase markedly at the corners of a projected image and thus one way of reducing aberrations is to include vignetting surfaces in a projection lens which selectively remove abberated light directed towards the corners of the image. Although successful in reducing aberrations, this approach lessens the amount of light which reaches the corners and thus produces an image whose brightness is lower in the corners than at the center. Users can interpret such reductions in intensity as indicative of a lower quality projection system. It is thus desirable to minimize the drop-off in light intensity at the corners of an image, e.g., at the full field in the long conjugate focal plane, introduced by the projection lens.
D. Cost
Moskovich, U.S. Pat. No. 5,625,495, entitled “Telecentric Lens Systems For Forming an Image of an Object Composed of Pixels,” and Kreitzer et al., U.S. Pat. No. 6,195,209, entitled “Projection Lenses Having Reduced Lateral Color for Use with Pixelized Panels,” the contents of both of which are incorporated herein by reference, describe the use of anomalous dispersion glasses (also known as “abnormal partial dispersion” glasses) and/or optical materials having particular Q-values to correct lateral color. The present invention employs the techniques of these patents, but with the added constraint that the cost of the lens is reduced while still maintaining high performance and, indeed, in some embodiments, performance levels not previously achieved even with more expensive designs.
The use of special glasses in a projection lens can rapidly increase the lens' cost, especially if such glasses are used in large lens elements. For example, the anomalous dispersion glass S-FPL51, which has a Q-value of 120.0, currently costs about $95 per pound. For comparison, standard flint and crown glasses, such as S-TIM2 and S-BSL7, currently cost around $30 and $10 per pound, respectively.
Cost not only rises when a glass with special dispersion properties is used, but also varies with the glass' index of refraction. For example, the breakpoint between inexpensive and expensive flint glasses is around N=1.9, with flint glasses having indices below this value being less expensive. For crown glasses, on the other hand, the breakpoint is around N=1.6, with crown glasses having indices above this value being less expensive.
Compared to glasses and, in particular, compared to glasses having special dispersion properties, plastic materials are generally much less expensive. In particular, as illustrated by the examples presented below, in accordance with the invention, it has been found that optical plastics, such as acrylics, can be used to provide high positive Q-values for the large lens elements of a projection lens, and thus low levels of secondary lateral color, at a fraction of the cost of a high +Q glass such as S-FPL51. The cost reductions achieved in this way are especially important for projection lenses that are to be used in consumer products, where price competition is most intense.
E. Telecentricity
The above-mentioned pixelized panels and, in particular, DMDs, typically require that the light beam from the illumination system has a near-normal angle of incidence upon the display.
In terms of the projection lens, this translates into a requirement that the lens has a telecentric entrance pupil, i.e., the projection lens must be telecentric in the direction of its short imaging conjugate where the object (pixelized panel) is located. This makes the lens asymmetric about the aperture stop which makes the correction of lateral color more difficult.
F. Cabinet Size
For rear projection systems, there is an ever increasing demand for smaller cabinet sizes (smaller footprints).
In terms of the projection lens, this translates into a requirement that the lens has a wide field of view in the direction of the image (screen). Increases in the field of view from, for example, 80° to, for example, 94°, can be of substantial significance to manufacturers of projection televisions. This is so because such an increase in the field of view of the projection lens can allow the TV manufacturer to reduce the dimensions of its cabinet by an inch or more. A smaller cabinet, in turn, makes a projection television more desirable in the highly competitive consumer market for PTVs.
The requirement for a large field of view makes it even more difficult to correct the lateral color of the lens. This is especially so when combined with the requirement for a relatively long effective back focal length which itself makes it more difficult to correct lateral color. Also, as mentioned above, the requirement for telecentricity is a third factor which makes the correction of lateral color challenging.
In addition to increasing the field of view, cabinet sizes can also be reduced through the use of a folded projection lens, i.e., a lens having an internal reflective surface (e.g. a mirror or prism) which allows the lens to have an overall form which is easier to integrate with the other components of the projection system and is more compact. In terms of lens design, the use of such a reflective surface means that two of the lens units making up the projection lens must be separated by a distance which is sufficiently long to receive the reflective surface. A construction of this type makes it more difficult to correct the aberrations of the system, especially if the lens is to include only a relatively small number of lens elements as is desired to reduce the cost, weight, and complexity of the projection lens.
Achieving a relatively long back focal length, a wide field of view in the direction of the lens' long conjugate, telecentricity, and a folded configuration, while still maintaining high levels of aberration correction with minimal vignetting and low levels of ghost generation, is particularly challenging since these various requirements tend to work against one another. To do so while reducing the overall cost of the projection lens is even more demanding. As illustrated by the examples presented below, the present invention in its preferred embodiments provides projection lenses which simultaneously satisfy these competing design criteria.